Thermal sensors are sensors that detect temperature changes at distant targets through the changes in radiation they emit. Since most objects can be treated as physical “black bodies,” the amount and the spectrum of radiation they emit strongly depend upon their temperature. The amount of radiation increases with temperature, and the peak wavelength of that emission decreases with temperature, such that objects at room temperature of 300° K. have a peak wavelength at the far infrared range of about 10 μm, while the Sun, with a surface temperature of 6000° K., has a peak wavelength at the visible green (0.5 μm). Therefore, sensors that are sensitive to radiation wavelengths corresponding to significant emission from the target objects can be used to detect changes in their temperature.
Using an array of such sensors, and placing it at the focal plane of an appropriate lens, the arrangement can be used to thermally map a scene, since each pixel in the array responds to a different area in the target. This concept is referred to as focal plane array (FPA), and is widely used in thermal imaging. Since most common objects have temperatures in the neighborhood of 300° K, infrared sensors are usually used for thermal imaging. Common thermal imaging applications include night vision, motion sensing, fire and smoke alarms, thermal mapping and control, and heat seeking applications.
Traditional sensors for the far-end of the infrared range are based on photon counting detectors. These detectors are based on narrow band gap semiconductors that have an energy gap corresponding to the far infrared photon energy. Incident photons are absorbed and their energy is used to generate charge carriers in the detector material. These charge carriers are converted to an electrical signal, such as a voltage or current, using the specific structure of the detector, e.g. a diode.
In order to implement a sensor array for imaging, an array of detectors is fabricated. Since electrical signals have to be read from all the sensor pixels of the array, an electronic readout circuit is used to serially select, amplify and signal process each sensor pixel signal. This circuit includes both analog and digital sub-circuits for operation. In recent years Complementary Metal Oxide Semiconductor (CMOS) technology emerged as the leading technology for digital circuits and as a result it also started to dominate a range of analog and mixed applications. Therefore, it is also found in many applications of thermal imaging. Thus, since the materials used for the readout circuit and the detectors are different, two separate microelectronic chips are fabricated and later flip-chip bonded to one another using conducting bumps that connect each sensor pixel to its dedicated circuit.
Sensors based on photon counting detectors are very sensitive, but suffer from several drawbacks. The major disadvantage is that they require significant cooling for their operation, typically down to cryogenic temperatures of about 77° K. As a result, the package of the sensor has to be evacuated to very a high vacuum, and additional cooling means need to be employed.
These demands, of course, increase the cost of the system, and its size and complexity. The use of exotic semiconductors for the detectors themselves, and the need to use two chips; and bonding them together, also increases the cost and complexity of the product, and reduces the yield and reliability of the system.
The alternative to using photon-counting detectors is to use indirect thermal sensors. Instead of directly converting each photon into an electrical signal, when using thermal sensors, incident radiation absorption causes a temperature change in a thermally isolated element. The temperature change is then converted into an electrical signal using a temperature sensitive electrical element. Such sensors that detect temperature changes can also operate efficiently at room temperature and do not require cooling.
The temperature change in uncooled thermal sensors is usually converted to an electrical signal using one of three methods. Thermocouples can be used to measure the temperature difference between the sensor and the ambient temperature, with low sensitivity to the ambient temperature and there is no need for an applied voltage or current. The common approach today utilizes resistive bolometers that measure the absolute temperature of a temperature-sensitive resistor. A third common option is pyroelectric sensors that change the charge in a capacitor in response to temperature changes.
The main disadvantage of uncooled thermal sensors has been the long response time, which is limited by the thermal time constants. The revolution of micromachining created a renaissance for uncooled thermal sensors since it enabled the fabrication of a sensor with a small thermal capacity and high thermal isolation, that give rise to high sensitivity and short response times appropriate for conventional video frame rates.
Conventional designs of uncooled FPA's include a CMOS chip for the array readout circuitry, on top of which a sacrificial layer and sensor material layers are patterned and then released using surface micromachining. The sensor materials include materials that are not part of the CMOS process and are usually considered unconventional in the microelectronics industry. As a result, these chips cannot be fully fabricated using standard CMOS lines, and post processing steps required after the standard CMOS fabrication may affect the CMOS devices performance due to non-compatibility. Yield and cost are therefore still a problem, while performance does not approach that of the traditional cooled sensors.
There are patents and publications regarding other types of uncooled sensors, such as microbolometers, pyroelectric sensors and thermoelectric sensors and also sensors that use diodes and thermo-mechanical devices for uncooled detection.
Therefore, there is a need to provide an array of uncooled infrared transistor-based sensors for use in micromachining and microsensors with improved cost, yield and performance characteristics.